First prepare your onions. Peel and thinly slice one onion and place the slices in a small bowl. Cut the other 2 onions in half through the root, keeping the root end attached. Gently trim any exposed roots, keeping the onion halves intact. Remove the peel and cut each onion half into 3 evenly sized wedges, being careful to keep the root end attached. Reserve the 12 onion wedges on your cutting board.

Spread your defrosted puff pastry on a sheet of parchment and prick all over with a fork. Let rest in the refrigerator until you’re ready to assemble and bake your tart.

Preheat your oven to 400 degrees. Melt 2 TB butter in a 9” oven safe skillet and set over low-medium heat. Add sliced onion. 3 sprigs thyme, and a pinch of salt. Cook over low heat, stirring frequently, for 10-15 minutes or until the onion slices are softened and a deep brown color..

Add sherry and balsamic vinegar to your caramelized onions set over medium heat and cook until no liquid remains. Remove the thyme sprigs and discard. Transfer your caramelized onions from the skillet to a bowl and melt the remaining 2 TB butter in the bottom of the warm skillet. Use a dish cloth or paper towel to spread the melted butter across the bottom and sides of your pan. Sprinkle the bottom of the pan with sugar.

Arrange your reserved 12 onion wedges in concentric circles or a pretty pattern in your pan. Fill in the gaps between wedges with cooked caramelized onions. Season lightly with salt and pepper. Top with ½ cup shredded imported Comte cheese. Carefully transfer your puff pastry sheet on top, trimming any excess pastry that hangs over the side of your pan and tucking the edges in around your onions.

Bake for 35-40 minutes or until your pastry is well browned. Place an upside down serving plate on top of the pan and using two oven mitts, quickly flip your pan upside down to unmold the tart onto your platter. (It's ok if some of your onion wedges shifted, you can just use a fork to rearrange!) Top the warm tart with remaining ¼ cup shredded imported Comte, garnish with remaining thyme, and serve warm or at room temperature, cut into slices.